Fluid processing systems that utilize fluid detecting probes have been in use in many fields. One of the more common types of fluid processing systems using a fluid detecting probe is a fuel burner type system in which a boiler is heated to generate hot water or steam for either heating or use in a manufacturing process. Normally the fluid sensing probe is a water level detector in the form of a metallic probe that is electrically insulated from the container or boiler. A conductive circuit is established between the probe itself and the fluid or water in the boiler to establish whether the fluid is at a proper level. In boiler water applications it is very common for the minerals in the water to form on the probe itself, and these minerals can change the characteristics of the probe in such a manner as to create an unsafe type of condition.
In order to monitor the condition of the probe and the water in a boiler some of the prior art systems have utilized test switches. In one prior art application the test switch removes the probe from the circuit and replaces it with an indicator light. The light functions to indicate whether or not a complete circuit exists through the probe to the water and then to the boiler itself. A further type of prior art test device utilizes a multiple contact switch which places an indicator light across the impedance of the probe and the boiler. If the probe was shorted out, the light will not be activated. If the probe is not shorted out, the impedance across the probe to the boiler ground acts to energize the light and indicates a safe condition.
In the systems just described electronic amplifiers are not utilized to improve the sensitivity of the system. The test switch functions previously described would monitor only the condition of the probe itself and would not reflect on the status of the overall system.
In the prior art, in order to provide a more reliable and fail safe type of system, a three element probe has been developed and is fully disclosed and described in the U.S. Pat. No. 4,027,172 to William B. Hamelink, and assigned to the assignee of the present invention. This three element type probe provides for a substantially fail safe type of boiler water monitoring system. While the probe itself is basically fail safe, there are certain types of installation conditions or faults which, if they exist, could mislead the user into the belief that the system was operating properly. Also, if the probe is used with an amplifier, there is no means disclosed in the prior art to verify the status of the amplifier as opposed to the probe itself.